Among the many fuels indigenous to the United States, coal remains relatively abundant while other fuels such as petroleum and natural gas become increasingly more scarce. If given a choice at competitive costs, most users will select natural gas or petroleum derivatives for fuel because coal is a dirty fuel that requires extra handling steps, and even then tends to have an adverse effect on the environment.
All three fuels serve a primary use in being burned for their heat content. Such burning normally is conducted with an abundance of oxygen, resulting in the carbon content being converted into carbon dioxide, the hydrogen content into water vapor and the sulfur content into sulfur dioxide. Among these flue gases water vapor is generally considered harmless unless it is expelled in such quantities as to change the climate in the local area. Carbon dioxide, a necessity to the growth of plant life in dilute quantities, may also have deleterious effects when discharged to the atmosphere in such quantities as to become a significant portion of the air. Sulfur dioxide can react with water vapor to form sulfurous acid, a product that can have serious effects on the environment. Further, a portion of the sulfur dioxide can further oxidize into sulfur trioxide which when combined with water vapor forms sulfuric acid mists which can cause disastrous environmental effects.
Generally, natural gas and petroleum derivatives have very small quantities of sulfur contents in the order of 0.1% or less, while coal commonly contains 0.5% sulfur or more. Thus much coal, which is otherwise suitable as a fuel, may not be used as a fuel because of environmental restraints. Coal suffers another drawback in that it contains a substantial amount of non-combustible material that becomes a residue of the fire as a dry solid of powdery material or as clinker. Coal residue, in addition to causing a handling problem, also in its disposal becomes an environmental problem. Depending on how coal is fed to the fire, some or most of the ash becomes particulate matter carried in the flue gas for dispersal into the atmosphere, if not otherwise intercepted by special equipment.
A considerable amount of research and development effort has been expended in recent years directed toward the removal of particulate matter and sulfur compounds from flue gases. A substantial amount of the particulate matter may be removed by one of several means well known in the art resulting in relatively modest costs of removal and disposal. Removal of the sulfur compounds at reasonable costs is not so easily done. The most promising schemes at the current state of the art require the addition of lime or limestone to react with the sulfur compounds and thus remove a substantial portion of the sulfur from the flue gas. Unfortunately the residue of these processes is a useless material which creates still another disposal and environmental problem.
The national energy policy of the United States currently is directed toward minimizing reliance on energy supplies located outside its sovereignty. A major effort is directed toward reinstating coal to a dominant position in the domestic energy supply. With an enormous investment in pipelines and equipment designed for the use of natural gas, both industry and the population in general are faced with further substantial investments in the conversion to alternate fuels such as coal. Added investment is only part of the problem because a basically dirty fuel requires cleansing somewhere along the line to avoid serious degradation of the environment.
In the use of coal the major thrust, in the short term, is directed to clean up after the fuel is burned. In this short term approach, the emphasis is on clean up of stack gases with only a minor amount of effort directed toward making continuous operations out of traditionally batch-type operations of conventional recovery and use of coal. Thus for many years into the future the preponderance of the production work force will be consigned to underground work stations executing the batch operations of grub, sort, convey, off load and hoist. Working conditions underground, though improved in recent years, involve both hazards from instantaneous accidents and from long term exposure to contaminated breathing environments.
Working conditions for coal miners are significantly improved when coal is recovered by open cast methods. The strip mine production workers, however, continue the batch operations practices of the past after the overburden is removed and the coal grubbed out: off-load, size, sort, pile, pick up and load on transporters. The newer and larger strip mines are located in western states where bulk transportation is normally in the form of unit trains that proceed loaded to the point of use and then return empty to the mine. Most of the towns in the western states owe their origin to the coming of the railroads. Such towns typically were built with the business district on both sides of the track, a convenience of the time that did not anticipate the unit train. Currently with a score or more of unit trains passing through a particular town each day the social impact is dramatically highlighted during a period of emergency such as a building on fire, with the fire trucks on the other side of the track while a unit train lumbers through the intersection for up to five minutes.
Alleviation of the coal transportation problem has been planned in the form of long distance coal slurry pipelines. This proposed solution is controversal in several respects, not the least of which is the requirement for water as the carrier liquid for the slurry. In the arid western states the use of potable water in a coal slurry is generally considered to be an unsatisfactory use of a scarce commodity. Substituting a coal-derived liquid, such as methanol, for the slurry would be considerably less controversal. The manufacture of methanol from coal provides other benefits such as diverting to other uses natural gas currently used as a feed stock for methanol synthesis. This would provide an additional supply of natural gas for the gas distribution pipelines.
The problem of an adequate supply of natural gas to fill interstate pipelines continues to be of grave concern to gas utilities with many pipelines operating at a fraction of their capacity. With natural gas prices controlled at artificially low levels, incentives for further exploration have been depressed to the point where demand has overtaken supply with resultant shortages. With improved prices the time lag for exploration and production will perpetuate shortages for many years in the future. Since there is no assurance that enough natural gas can be discovered to satisfy demand, it would appear prudent to develop sources of synthetic natural gas (SNG) independent of conventional petroleum.
An interim solution to the natural gas shortage has been undertaken in the form of imported liquefied natural gas (LNG). Aside from the problem of a distant source, LNG introduces other problems in that to become liquid, natural gas must be cooled below its vaporization temperature, an inconvenient temperature in the order of -260.degree. F. At this temperature natural gas is a liquid occupying approximately 1/600th of its original volume, a more convenient size for long distance transportation. Compacting energy in this manner introduces environmental hazards, particularly fire hazards should a rupture occur. One such disaster has already occurred in the U.S., with a substantial loss of life and property.
It would appear that a better approach to the natural gas supply problem is the synthesis of SNG from coal. Several projects have been proposed that would use the well known Lurgi system for gasifying coal in above ground facilities. Such a system relies on mining coal in the conventional manner, crushing coal to a predetermined size, then introducing the sized coal into the gasifier. The resultant SNG is readily interchangeable with natural gas of petroleum origin. Sizing the coal generates a substantial amount of fines that are unsuitable for the Lurgi system, therefore a market must be found for the fines, logically an adjacent coal-fired steam electric generating plant. The coal mine, the Lurgi plant and the electric plant place a heavy load on the environment in the general area of their sites. The Lurgi plant, with a water requirement of approximately one pound for each pound of coal consumed, together with the electric plant and its requirement for water results in a substantial withdrawal from the local water supply.
An even better approach would appear to be the synthesis of SNG from coal in situ. Such an approach avoids the problems of upheaval of the topsoil inherent in strip mining, the hazards of man power underground in conventional deep mining, the ash disposal problem inherent in consuming coal above ground, and the like. There are many coal deposits in the western states that lend themselves to in situ techniques, a method of producing coal that has been in commercial practice in Russia for several decades. While the Russian approach to producing coal in situ is not known for synthesis into SNG, opting instead for low BTU gas as a fuel, innovations to the Russian system can produce synthesis gas which is readily converted into SNG using well established technology.
Water requirements for the Lurgi system are dictated in part by process needs and in part by the need to keep metal parts within temperature limits. Since the in situ reaction zone is within the coal bed underground there is generally no requirement for water to limit temperatures. Thus there are many western subbituminous coals with relatively high water content in the coal itself that satisfy the process needs for water without a requirement for outside supplies.
In situ techniques are deceptively simple, a fortuituous circumstance since the reaction zone is underground away from the eye of the operator. With a simple process, considerable latitude is granted in the control of the desired processes. The processes may be conducted sloppily as compared to aboveground processes, yet be conducted safely and within planned tolerances. While in situ techniques do not eliminate environmental problems a proper practice of in situ techniques in concert with aboveground techiques can minimize environmental impacts as compared to other methods of recovering and utilizing coal.